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Dec. 21, 1943. vo Re. 22,413

TELEPHONE SYSTEM Original Filed Feb. 7. 1938 14-Sheets-She et 2 FIG. 3-

@SUBSCRIBEg LINE CIRCUIT f l SUBSCRIBERS [NE CIR UIT 2 2 AUTOMATICEXCHANGE TRUNK I INE cmc urr I I RA392 INVENTOR. JOHN H. VOSS "M, M m

ATTORNEY .5,

Dec. 21, 1943.

Original Filed Feb. '7. 1938 J. H. VOSS TELEPHONE SYSTEM 14 Sheets-Sheet3 JOHN H. VOSS BY m ATTORNEY5.

Deg. 21, 1943. .J.H. voss TELEPHONE SYSTEM Original Filed Feb. 7, 193814 Sheets-Sheet 7 mmhu Dec. 21, 1943. J. H. voss Re. 22,413

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ATTORNEYS.

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INVEN IOR. Jol -1N H. VOSS WW, M M

ATTORNEYS.

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Dec. 21, 1943. J. H. voss TELEIHONE SYSTEM Original Filed Feb. 7, 193814 Sheets-Sheet 14 mmiu 330 mmEu mwio

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mwiu 7 Q i E H i i m a i i jooim INVENTOR. JOHN H. V055 BY wm m MATTORNEYS Reismed Dec. 21, 1943 can:

ramrnonn srs'rniu John H. Von, Downers Grove, Ill, assignor to AutomaticElectric Laboratories, Inc a corporation of Delaware Original No.2,252,809, dated August 12, 1941, Se-

rlal No. 189,121, February 7, 1988. Application for reissue January 25,1943, Serial No. 478,549

110 Claims.

The present invention relates to telephone systems and has for one ofits objects the provision of improvements in small automatic systems ofthe type adapted for installation in small towns or villages.

An automatic telephone system of the character noted is generally knownas a community automatic exchange or C. A. X and usually has a linecapacity of one hundred lines or less. Certain of the subscribers linesmay be oi the metallic, two-conductor variety while others of thesubscribers lines may be of the well-known ground return type. Themetallic lines are usually of the multi-party type and bridged ordivided code ringing is utilized for selectively signaling thesubstations thereon. In order to provide toll service to thesubscribers, provisions are also made for trunking calls to and irom oneor more distant exchanges, One improved allrelay exchange of this typeis disclosed and claimed in the copending application, Serial No.158,443, Frank Kessler, filed Feb. '3, 1938. and the present inventionis directed more particularly to further improvements in the circuitsillustrated and described in this copending application. In brief, inthe system as disclosed in the Kessler application, the subscriberslines and trunk lines of the system extend to the finder and connectorswitches of each of a plurality of all-relay finder-connector links,common equipment including a relay type linkdistributor or allotterbeing provided for assigning idle ones of the links to successivecalling ones of the lines in a definite order. The finder switches ofthe various links are fully directive in character; that is, all lineshunting operations are eliminated through anarrangement whereby only thegroup and unit relays corresponding to a particular calling line arecaused to operate when a link is taken for use. For the purpose ofdecreasing the cost of the links, the number of unit relays provided inthe finder and connector portions of each link are reduced by halfthrough the provisions of a subgroup relay. By providing such subgrouprelays, each unit relay of each line selecting finder or connectorswitch is rendered operative to select either of two lines dependingupon the operated or non-operated condition of the associated subgrouprelay. In the particular disclosed arrangement of the finder switches,however, there is a possibility of a busy line being momentarilyconnected through to the link circuit of a seized link. to the annoyanceof the conversing parties, due to the fact that no apparatus is providedfor insuring the operation of the subgroup relays before the operationof selected ones of the unit relays. In the improved trunk selectingfacilities of the system disclosed in the above-referred-to copenclingapplication, no provisions are made for fully utilizing the unit relaysof the various connector switches to accommodate the selection of trunksarranged in groups of more than five lines, these relays only beingindividually operative to select for use a single corresponding trunkwhen the corresponding trunk is idle. Also, no distinction is madebetween the trunk lines and the subscribers lines as regards the methodof arranging the line circuits individually associated therewith.Provisions are made for the selection of coded ringing current forsignaling desired substations on the multi-party lines, the codeselection being entirely performed in any operating link through theresponse of the link circuit apparatus to a code selecting digit whichfollows the line selecting digits required for the selection of thedesired called line.

It is a further object of the present invention to provide for use in asystem of the character noted an automatic line selecting switch of theail-relay type which is fully directive in character, which requires aminimum number of relays both in the switch itself and in the associatedcommon equipment, and which is operative to preclude the connection ofmore than one line to its associated link circuit during a lineselecting operation.

It is another object of the invention to provide an improved arrangementof the allotter or distributor equipment associated with the all-relayfinder-connector links, whereby a new link is substantiallyinstantaneously assigned to the use of a calling line in the event afirst assigned link fails to find a calling line, and which functions ina manner such that there is no possibility of more than one link seizingthe same calling line at the same time or more than one calling linebeing seized by one link.

It is a further object of the invention to provide an improved lineterminating circuit which is so arranged that the line associatedtherewith is immediately marked as busy in the connector switches towhich it extends when a calling condition is created on the line andbefore the finder switch of an assigned link functions to seize theline.

It is an additional object of the invention to provide an improvedfinder-selector link circuit arrangement whereby either of two diiferentline circuit arrangements may be employed in terminating difierent onesof the lines extending to the link.

It is a further object of the invention to provide in a link 01 thecharacter described an improved arrangement for determining the type 01ringing to be utilized in ringing over a selected called line and fordetermining the code 01 the ringing current to be projected over theselected called line.

It is a further object of the invention to provide, in a link circuitarranged to provide divided ringing on multi-party metallic lines, anarrangement for preventing the ringing current source from beingshort-circuited when ringing over a selected line or the ground returntype.

It is another object of the invention to provide improved apparatus forregistering the operation of a link each time the link is involved inaconnection.

It is a still further object of the invention to provide an improvedarrangement for selecting idle ones of a group of trunk lines accessibleto a link, which arrangement is such that the number of selecting relaysutilized in each of the several links is reduced to a minimum.

The features of the present invention are illustrated as beingincorporated in an exchange having an ultimate capacity or one hundredlines, ten of which are two-way trunk lines extending to a distantexchange and the remaining ninety of which are subscribers lines. Thevarious lines individually terminate in line circuits which aredifferently connected and arranged depending upon whether the terminatedline is a trunk line or a subscriber's line. In accordance with onefeature of the invention, the control apparatus of any assignedfinder-connector link is so arranged that the line circuit associatedwith any calling one of the lines is caused to operate when the assignedlink finds the calling line irrespective of which type of line isresponsible for the seizure of the link. More specifically, each linecircuit which terminates a subscriber's line is of the usual typewherein busying potential is only applied to the private or controlconductor of the line following the operation of an assigned link toseize the associated line when a calling condition is created thereon.In each of the line circuits terminating a trunk line, an improvedarrangement is utilized which insures that the terminated line will bemarked as busy immediately a calling condition is created thereon. Tothis end, each of the last-mentioned line circnits has embodied thereina line relay which operates to apply a predetermined busying potentialto the private or control conductor of the terminated line when a callis initiated on the line. Also, each 01 the last-mentioned line circuitsis provided with a cut-oil relay which is operative to disconnect theassociated line relay from across the associated line only in responseto the application of a, predetermined potential to one of the talkingconductors of the associated line. In order to make each link circuituniversally efl'ective to control either form line circuit, there isprovided therein control apparatus for applying busying ground potentialto the control conductor 01' a seized calling lin and also to the linecondoctor of the link line by way of which the cutoff relays of thetrunk terminating line circuits are energized.

For the purpose of selecting idle ones of the links for the use ofcalling ones of the lines and for the Iurther purpose of controlling theoperation of the finder switch of any assigned link to seize the callingline to which it is assigned, there is provided in association with theseveral links common equipment including a group of allotter relaysindividually corresponding to the links and a group of auxiliary relaysindividually corresponding to the unit relays oi. the respective finderswitches. The allotter relays are connected and arranged sequentially tooperate to select idle ones 0! the links and are individually providedwith looking or holding circuits which are so arranged that th lockingcircuit for each relay is opened in response to the operation of thenext succeeding relay. The allotter relays are arranged to control startcircuits individually extending to the various links. These startcircuits are successively prepared during the sequential operation 01'the allotter relays and an improved arrangement is provided formaintaining the start circuits open independently of the allotter relaysduring the sequential operation of these relays. In order to preventmore than one of the allotter relays from inadvertently being lockedoperated in a link assigning position at the same time, there isprovided an additional relay which is operative to prevent two 01' thelocking circuits from being held completed at the same time for aprolonged time interval. Provisions are also made in the allotter forassigning a new link to the use of a calling line in the event a firstassigned link fails to find the line. To this end and in accordance witha further feature of the invention, there is provided a relay which isconditioned to operate from one position to a second position inresponse to the assignment of one of the links to the use oi aparticular calling 'line. and the allotter relays are arranged torespond to the operationsoi this relay to its second position to assigna second link to the use of the calling line. This relay is constructedto have an operating characteristic such that the time required for itto operate from its first to-its second position, following theconditioning there- 01 for such operation, exceeds the time normallyrequired for the finder switch of one o! the links to seize a callingline. The circuit arrangement is such that ii the first assigned link issuccessful in finding the calling line the relay is prevented fromoperating to its second position.

The arrang ment oi the unit relays provided in each 01' the finder andconnector switches is similar to that employed in the system of theabove-referred-to copending application in that the lines of each linegroup are divided into subgroups and each unit relay is operative toselect either of two different lines individually included in diiterentsubgroups depending upon the operated or non-operated condition of anassociated subgroup relay. In accordance with a further feature oi! thepresent invention, however, the operation 01' any unit relay provided inany one 01' the finder switches and corresponding to a line to beselected is made dependent upon the prior operation of the associatedsubgroup relay in any case where the selection of the line requires thatboth the subgroup relay and th unit relay, corresponding to the line, beoperated. More particularly, each finder switch is arranged so that thesubgroup relay provided therein operates when a. calling line isincluded in one subgroup of lines and, upon operating. completes theoperating circuit for the one unit relay which is operative to selectthe calling line, whereas this one unit relay is caused to operateindependently of the subgroup relay when it is included in the othersubgroup of lines. By this arrangement there is eliminated anypossibility of a busy line momentarily being connected through to thelink circuit of an assigned link.

In accordance with a still further feature of the invention theconnector switch of each link is provided with a subgroup relay which isconnected and arranged to cooperate with the unit relays of the switchin selecting idle ones of the group of ten trunks terminating at theswitch. More particularly, two sets of relays are provided inassociation with the trunk lines, which relays are so arranged that anidle trunk line is selected immediately a previously selected trunk lineis made busy. The function of any one 01 the connector switches on atrunk call is, therefore, to pick out a trunk which has previously beenselected by the two sets of relays noted. To this end. the controlapparatus provided in each link is arranged to cause the operation ofthe one of the unit relays which is operative to select a preselectedidie trunk, without causing the operation of the subgroup relay, so longas certain of the trunk lines are idle, and is arranged to cause theoperation of both the subgroup relay and one of the unit relays whenmore than a predetermined number of the trunk lines are busy.

Each link is provided with a group of impulse counting relays whichfunction to control the connector group and unit relays during theoperation Of the connector portion of the link to select a desiredcalled line. Two successive operations are required on the part of theimpulse counting relays to cause the selection of a desired calledsubscriber's line, the first line selecting operation of the countingrelays being eflectlve to cause the selection and operation of the grouprelay in the connector switch which corresponds to the particular groupof ten lines including the called line, and the second operation beingutilized to cause the selection and operation oi the particular unitrelay corresponding to the called line, or the operation of this relayand its associated subgroup relay. For the purpose of preventing theringing current source from being short-circuited in ringing over asubscriber's line of the ground return type, improved apparatus isprovided for preventing the ground conductor of a line of this type frombeing connected to the ungrounded output terminal of the source duringthe ringing operation. At least a, portion of the subscribers lines areof the multi-party type and certain thereof are arranged for dividedcode ringing. Others of the multi-party lines are arranged for bridgedcode ringing. This means that each link must be equipped to provide coderinging of either type and that suitable provisions be made forselecting the type of ringing as well as the particular code of theringing current to be projected over the called line. In accordance withanother feature of the invention, apparatus is provided in each linkwhich responds to the operation of the associated impulse countingrelays during the dialing of a digit preceding the line selecting digitsto determine whether bridged or divided ringing will be utilized inringing over a selected one Of the subscribers lines. This apparatusalso functions at least partially to select the code of the ringingcurrent to be projected over the selected line. More specifically. theoperation of this apparatus is dependent upon the number of impulsescomprising the first digit dialed into the counting relays from acalling substation and, if the digit comprises ten impulses. it does notoperate. In the case of a ten-impulse first digit, certain 01 thecontrol apparatus embodied in the link circuit functions to cause apreselected idle one of the trunk lines terminating at the connectorportion of the link to be selected for the use of the calling subscriberby the connector portion of the link.

7 For the purpose of registering the operation of the links, there areindividually provided therein registering or recording devices, each ofwhich is arranged to operate only in response to the operation of theassociated counting relays when a first-received series of impulsestransmitted thereto comprises more than one impulse. By this arrangementthe operation of a link is only recorded when it is taken for use by acalling line and a series of impulses is actually transmitted theretofrom a calling substation associated with the calling line.

Fm'ther features of the invention pertain to the particular arrangementof the circuit elements whereby the above-outlined and additionaloperating features are attained.

The novel features believed to be characteristic of the invention areset forth in the appended claims. The invention, both as to itsorganization and method of operation, together with further objects andadvantages thereof, will best be understood by reference to thespecification taken in connection with the accompanying drawings inwhich Fig. 1 illustrates the general arrangement of a system to whichthe present invention pertains, Fig. 2 illustrates the mode of combiningthe succeeding figures 1' the drawings to form a unified system, andFigs. 3 to 14. inclusive, illustrate the details of a system havingincorporated therein the features of the invention as briefly outlinedabove.

Referring now more particularly to Fig. 1 of the drawings, the systemthere shown comprises a small community exchange Ill and a secondautomatic exchange 30 interconnected by a group of two-way trunk lines,one of which is indicated in the drawings at It" as terminating at a.repeater circuit 20 forming a part of the apparatus located at theexchange l0. Also extending to the exchange III are a plurality ofsubscribers lines, three of which are indicated at H, i2 and it. Each ofthe enumerated subscribers lines is illustrated as being of themulti-party type having two or more telephone substations associatedtherewith. Thus, two telephone substations A and F are diagrammaticallyshown as being connected to the line H, two additional substations B andC are shown in association with the line II and two substations D and Eare illustrated as being connected to the line it. Each of thesubstations connected to the various subscribers lines may comprise theusual hand set telephone having embodied therein a dial controlledimpulsing device of conventional arrangement. Each of the linesextending to the exchange Ill terminates in a line circuit through whichit is rendered accessible to the finder and connector switches includedin the exchange. Thus, the line H terminates at a line circuit 2ithrough which it is connected to the contact springs of a finder switchII and a connector switch 26. which two switches are connected togetherto form a finder-connector link. Similarly, the line l6 terminates at aline circuit 22 and the illustrated two-way trunk It interconnecting theexchanges it and 30 terminates at a line circuit 23.

The exchange II is designed to serve a relatively small number ofsubscribers lines, specifically 90, and the trunk lines of a ten-linetrunk group and, accordingly, all of the subscribers lines and trunklines are arranged to be served by any one of the plurality offinder-connector links included therein. Obviously, the number of linksrequired for satisfactory service will depend upon the volume oftelephone traffic but, taking into account the character of theexchange, ten links are considered adequate for reasonably good service.In order to assign the links to successive calling lines in a definiteorder so that a link is always available for the use of a calling line,excepting. of course, those instances when all of the links aresimultaneously busy, there is provided certain common equipment 25 whichincludes a link distributor or allotter. The several links also havecommonly associated therewith ringing, tone and timing apparatus whichis diagrammatically shown at 21;

Neglecting for the present the details of the circuit arrangement, andconsidering briefly the operation of the system, when a subscriber,such, for example, as the subscriber at the telephone A, initiates acall over the line I l to a second subscribers telephone, such, forexample, as the telephone C, by lifting his receiver from its hook, theline circuit 2| operates to cause the finder-connector link previouslyassigned by the distributor to the calling line, to be started inoperation to seize the calling line. Assuming that the link 24, 2B isdelegated by the distributor to set up the connection, the finderportion 2! of the link operates to select the calling line and toconnect it through to the conductors of the link line incoming to theconnector portion 26 of the link. When the calling line H is seized andextended by way of the finder switch to the associated connector switch,it is marked as busy in all of the other connector switches and thecontrol equipment of the seized link is automatically conditioned torespond to the impulses of the first series of impulses transmitted fromthe impulsing device at the calling substation A. Since the subscriberslines are of the multi-party type, provision must necessarily be madefor selectively signaling the desired called substation on any selectedline. With the arrangement to be described more in detail hereinafter,coded ringing is utilized to accomplish this end. It is furthercontemplated that certain of the lines may be arranged for bridgedringing and others of the lines may be arranged for divided ringing and,in accordance with one feature of the present invention, the linkcircuit apparatus of any seized link is arranged to respond to theimpulses of the first digit dialed at the calling substation to selectthe type of ringing to be used and further, if bridged ringing isselected, partially to select the code of the ringing currentsubsequently to be projected over the called line. Further in accordancewith this feature of the invention, the operation of the link circuitapparatus during the first digit determines whether the selected linewill be a called subscribers line or a trunk line extending to thedistant exchange. Also, if the number of impulses making up this firstdigit is more than a predetermined number, a registering device providedin the link is caused to operate to register the operation of the link.

In the case under consideration wherein the call is to be extended tothe subscriber's line It, the connector switch 26 will respond to theimpulses of the first digit to select the type of ringing to be utilizedand to cause the operation of the link to be registered or recorded. Ifthe called line It happens to be one of the lines arranged for bridgedringing, the first digit also results in the partial selection of theringing code to be used in ringing over the called line.

The connector switch 26 responds to the impulses of the second digitdialed at the calling substation by selecting the group of ten lineswhich includes the called line It. Thereafter, and in response to thedialing of the third digit, the called line I6 is selected from theselected group of lines. When the called line is selected. the controlapparatus included in the link circuit functions to test this line forthe purpose oi determining the busy or idle condition thereof.

Busy tone current, as generated by a portion of the common apparatus 21,is not returned over the p extending to the calling subscriber'ssubstation until the fourth or code selecting digit is dialed at thissubstation. When this digit is dialed, the link circuit apparatusfunctions to select the particular code of the ringing current to beprojected over the called line for signaling the substation desired bythe calling party. At the conclusion of this digit and if the calledline is busy, busy tone current is returned over the loop to the callingsubscriber's substation to indicate to the calling party that thedesired connection can not immediately be completed. Ii, on the otherhand, the called line is idle at the time it is selected, ringingcurrent of the selected code corresponding to the called substation C isprojected over the called line for the purpose of energizing thesignaling devices individually provided in association with at least aportion of the substations on the called line. In this connection itwill be understood that, if divided ringing is utilized, the ringingcurrent is projected over the side of the line to which the ringingdevice at the desired substation is connected. On the other hand, ifbridged ringing is selected as the type of ringing to be used, ringingcurrent is conducted over both sides of the called line to energize allof the ringing devices individually provided at the substationsconnected to the line. During the ringing operation, ring-back tonecurrent is applied to the loop extending to the calling substation toindicate to the calling party that the desired substation is beingsignaled. Finally, a connection between the calling and calledsubstations is completed when the call is answered at the substation C.As pointed out with particularity hereinafter, the arrangement of thelink circuit apparatus is such that a link utilized in setting up adesired connection is released only when the connection is cleared outat the calling substation.

The switching apparatus located in the exchange Ill may also be utilizedin setting up a connection between two substations connected to the sameline, the sole function of the apparatus in this regard being that ofselecting and applying ringing current to the calling line of the propercode to signal a party at the called substation. The discharge of thisfunction necessitates the seizure of the calling line by an idle link,all in the manner outlined above. Thereafter, and following the dialingof the four digits of the called substation directory number at thecalling substation, the calling party restores his receiver to its hook.When the calling party hangs up, the finder portion of the operated linkis released, and ringing current of the selected code identifying thecalled substation is projected over the calling line. If the callingline is one of the lines arranged for divided ringing, the link circuitapparatus functions to project a reverting call signal ringing currentover the line conductor which is not used for conducting ringing currentto the signal device at the called substation. This reverting callsignal ringing current energizes the ringing device at the callingsubstation to indicate to the calling party that the called substationis being rung. When the call is answered, the operated link is releasedand the line circuit 01' the line in service operates to busy the line,thereby to prevent its seizure by the connector portion of another linkon a second call to one or the substations on the line. Finally, theline circuit 01' the line in use is restored to normal when theconnection is cleared out at the calling and called substations.

In the operation of the link to select one of the group 01 trunk linesinterconnecting the exchange it and the exchange 38, only one digitcomprising a predetermined number of impulses is dialed at the callingsubstation. The connector portion or the seized link responds to theimpulses of this one digit to select a preselected idle one the group oftrunk lines. More specifically, these lines have associated therewithtrunk preselecting apparatus which functions to select an idle line foruse immediately a previously selected line is taken for use. Hence, atthe conclusion of a trunk selecting digit dialed'at a callingsubstation, the connector portion or the seized link immediatelyoperates to seize the preselected idle trunk. When the trunk line isseized, the calling line is switched through by way of the selectedtrunk line and its associated repeater circuit to the terminatingapparatus located in the distant exchange. In this connection, it isnoted that two sets of link line conductors are provided in the linkcircuit or each link, one or which is devoid of all voice frequencycoupling plements, such, for example, as condensers or repeating coils,and is utilized in extending a desired connection by way or one of thetrunk lines. Following the extension of the connection to the trunk lineterminating apparatus located in the distant exchange 30, the connectionmay further be extended to the desired called line through the dialingof the regular directory number corresponding to the desired line at thecalling substation. The manner in which the connection is releasedfollowing the establishment of a connection to the desired line dependsupon the nature of the repeater circuit associated with the trunk linewhich is used in setting up the connection.

Since the trunk lines interconnecting the exchanges HI and 30 are of thetwo-way type. it will be understood that any one of these lines and theswitching apparatus included in the exchange Ill may be utilized in thecompletion of calls originating or extended by way of the exchange 30and which are to be extended to one of the subscriber's linesterminating at the exchange Ill. The mode of operation of the equipmentin the exchange HI following the extension of a call thereto by way ofone of the trunk lines noted is substantially similar to that set forthabove with reference to the connection established between thesubstations A and C, it being observed that each of these trunk linesterminates in a line circuit connected and arranged in a novel mannerwhich is diflerent from the arrangement or the line circuitsindividually terminating the connection is entirely under the control ofthe calling party whose line terminates at the distant automaticexchange.

Considering now in greater detail the apparatus embodied in the exchangeIn and involved in the above-described sequence of operations, andreferring to Figs. 3 to'l4, inclusive, there are illustrated theelements of the line circuits respectively terminating the lines II, Itand 50, of the finder-connector link 24, 26, and of the common equipment25. There are also diagrammatically illustrated those portions of theapparatus 21 which are essential to an understanding of the operation ofthe system. The line circuits individually terminating the subscriberslines are of more or less conventional arrangement. More particularly,the line circuit 2| comprises a line relay R300, a cut-ofi relay R3"),and a line lockout relay R320 connected and arranged to operate in theusual manner. The corresponding relays for the line circuit 22 areindicated at R300, RSHI' and R320. This last-mentioned circuit isindicated as having associated therewith a pay station tone source 368which is connected through a condenser "I to impress a pay station tonecurrent on the control or private conductor C306 01 the line It for thepurpose of indicating to a supervising operator in a distant exchangeas, for example, the exchange 30, that the substation B is of the paystation type. This pay station tone circuit is shown in association withthe line It for illustrative purposes only, it being pointed out that acircuit of this form is provided only in conjunction with those of thelines terminating at the exchange III which are of the single party paystation type. Since the apparatus for applying pay station tone currentto the control conductor C306 may be entirely conventional in character,it has been only diagrammatically illustrated.

The line circuits individually terminating the respective trunk linesare identical in character and are diil'erently connected and arrangedfrom the line circuits terminating the subscribers lines. In brief, theline circuit 23 comprises a line relay R350, a cut-off relay R390, alookout relay R340 and a switching relay R330. The primary diiierencebetween this line current and the subscribers line circuit 2|, forexample, pertains to the method of busying the associated lines to callsincoming thereto. With this illustrated circuit arrangement of the linecircuit 23, the line relay R350 is operative to impress a line busyi gpotential on the private or control conductor C359 immediately a callingcondition is created on the associated line, and the cut-oil relay R390is operative over a circuit including the positive talking conductorC351 0! the line. With the subscribers line circuit 2|, on the otherhand, a line busying potential is only applied to the control conductorC306 following the operation of one of the finder switches to seize theline, and the cutoffrelay R3 I ll is only operative when the busyingpotential is applied to the control conductor C306.

Each of the finder switches is of the fully directive all-relay type,group and unit relays being provided for performing the line selectingfunctions. The lines terminating thereat are arranged in groups oi tenlines and the lines of each group terminate at the contact springs ofone group relay. More particularly, the finder switch 24 comprises tengroup relays corresponding to and including the two relays R318 and R380individually having contact springs for terminating the conductors often lines. While the group relays may be or any desired commercial typehaving the required number 01' contact springs, preierably these relaysare of the improved i'orm disclosed and claimed in the copendingapplication Serial No. 109,813, Herbert F. Obergi'ell, flied November 9,1936. The lines of each group are divided into two equal subgroups andthrough the provision of a subgroup relay in each finder switch a groupor live unit relays is suilicient to permit the selection of any desiredone of a selected group of ten lines. Thus, the tinder switch Mcomprises a subgroup relay R410 and live unit relays corresponding toand including the two relays R400 and Bill, each of which unit relays isoperative to select either or two lines in a selected group of ten linesdepending upon the operated or non-operated condition of the subgrouprelay Rllll. The finder switch 24 also includes a pair 01 switchingrelays R440 and R! which are arranged to be controlled by way or thecommon equipment illustrated in Figs. 5 and 6 and are operative toinitiate the operation of the finder switch to seize a calling line.

The common equipment shown in Fig. 5 includes a plurality of auxiliaryrelays which are arranged to be energized over start circuits controlledby the line circuits associated with the respective lines terminating atthe exchange l0. These relays are selectively operative in response tothe selective completion oi the start circuits and individually iunctionto condition for operation a particular group relay and a particularunit relay embodied in the finder portion of an assigned link. Moreparticularly, ten auxiliary group relays including the two relays R5"!and R5" are provided, which individually correspond to the finder grouprelays oi each link. A group of five auxiliary unit relays, includingthe two relays R530 and R5, are similarly provided which individuallycorrespond to the unit relays provided in the respective finder switchesand are operative to condition the corresponding linder switch unitrelays for operation. The energization oi the subgroup relay in thefinder portion of any assigned link is dependent upon the operated ornon-operated condition oi a subgroup relay R5 provided in the commonequipment, this latter relay also being arranged to be energized overthose of the start circuits which are adapted to be completed in theline circuits terminating the lines included in predetermined ones ofthe line subgroups. Specifically, the arrangement oi the apparatus issuch that when a calling condition is created on one oi the linesterminating at the exchange, start circuits are completed by the linerelay associated with the calling line for causing the operation of theone of the auxiliary group relays R5", R520, etc., which corresponds tothe group 01 lines including the calling line and for energizing the oneof the auxiliary unit relays R530, R540, etc., which is operative toselect the calling line. It the calling line is in one subgroup of itsparticular group the auxiliary subgroup relay R500 is not energized overthe completed start circuits, but it the calling line is in the othersubgroup, R500 is energined and operates to condition the subgroup relayembodied in the finder switch 01' the selected link for operation. Inorder to give the finder switchotanyassignedllnkampletimetoseizethecalling line to which it is assignedand to connect the line through to the connector portion of the linkbefore the auxiliary group and unit relays are reconditioned to respondto another call, the slowto-release relay RIM is provided. which relayis operative immediately a link starts to seize a calling line andfunctions to open the operating circuits for all of the auxiliary grouand unit relays.

The link alloter or distributor is illustrated in Fig. 6, as comprisinga group of ten allotter relays R810, R610, R880, R890, etc., whichindividually correspond to the ten links corresponding to and includingthat illustrated, an all-links busy relay RIOI, a link start relay R604and six additional relays Rilll, R620, R830, R640, R650, and R680. Ofthese six additional relays, the slow-to-release relay Bill and therelay R680 are provided for the purpose of opening the locking circuitsior all of the allotter relays when two or more 0! the allotter relaysare inadvertently locked operated in the link assigning position at thesame time, and for automatically causing a single allotter relaycorresponding to an idle link to be operated to its link assigningposition following interruption of the allotter relay locking circuits.The relay R820, the slow-to-release relay R630, and the slow-to-operaterelay R840 cooperate to cause the allotter relays to assign a secondidle link to the use of a calling line in the event a first assignedlink is found defective and fails to operate properly in response to theoperation of the start relay R604. The relay R628 also functions tocontrol the all-links busy relay R"! in a manner such that a multipleholding circuit is provided for maintaining RG00 operated during theoperation of an assigned link to seize the calling line, which holdingcircuit is opened to permit R600 to restore following the completeoperation of the assigned link to seize the calling line and in theevent the assigned link is the last available idle link. In addition,the relay R62! functions to hold open the link start circuitsindividually extending to the various links during the operation of theallotter relays to select an idle link. In order to insure the continuedoperation of R500 for a short interval after the allotter relays haveoperated to select an idle link, the slow-to-release relay R6" isprovided.

The link circuit proper is illustrated in Figs. 7 to 10a of thedrawings. The link line "I incoming to the link circuit terminates in arepeater I and the talking conductors thereof are arranged to beincluded in the operating circuits for two line relays R'Hil and Rlli.The relay RHll is the usual impulse responsive relay and in combinationwith the two additional relays R1" and R forms an impulse correctingnetwork for repeating impulses transmitted thereto from a callingsubstation over an impulsing circuit to the slow-to-release controlrelay R860 and the impulse counting relays illustrated in Figs. 10 and10;. The link hold relay is indicated at R180. this relay being arrangedto be held energized by a slow-to-release slave relay R150. The relayRlllli is the usual switching through relay which is energized only whenthe link is utilized in extending a connection by way of one of thetrunk lines to the distant exchange 30. The battery reversing relay isindicated at R115, this relay being arranged to operate in the usualmanner when a call is answered at a called substation, and functions tocontrol the operation 01 a slave relay R which actually performs thebattery reversing operation. Any selected called line is electricallybalanced through the provision of an impedance element ill having animpedance substantially equal to that of the battery reversing relayR115. For the purpose of preventing the link from being permanently tiedup through seizure by a defective line or a line having a call-

